This invention relates to a rotary drum type device for separating solid particles from a liquid.
Known in the art of separating solid particles from liquids such as sewage sludge and contaminated liquid is a vacuum filtration system using a vacuum dehydrator. The vacuum dehydrator has a rotary screen drum on which a filter cloth is attached. The rotary screen drum is partly submerged in a container containing a liquid to be treated and pressure of the inside of the rotary screen drum is reduced by operating a vacuum pump while the rotary screen drum is rotated. The liquid to be treated is thereby dehydrated and cake of solid particles is produced on the surface of the filter cloth and this cake is scraped off at the end of one cycle of the rotary screen drum by means of a scraper.
The prior art vacuum dehydrator however has the disadvantage that meshes of the filter cloth used as an element for separating solid particles from a liquid tend to be filled with solid particles during each cycle with resulting prompt reduction in filtration efficiency.
There has been proposed, as disclosed in U.S. Pat. No. 4,407,720, a rotary drum type device for separating solid particles from a liquid which includes a rotary screen drum in which the conventional filter cloth is replaced by a wedge wire provided on the periphery of the screen drum.
It is however impossible for this prior art device in which the screen surface is formed by wedge wire to realize a fine slit of less than 150 microns between adjacent wedge wire sections by reason of manufacturing technique and therefore it is only possible for this type of device to filter out solid particles having a diameter exceeding 150 microns. In case it is necessary for this type of device to separate solid particles having a diameter of less than 150 microns, an extra filter material such as a filter cloth must be attached on the wedge wire screen surface as described in column 1, lines 18-24 and column 2, lines 64-66 of the above mentioned U.S. Pat. No. 4,407,720. In this case, the problem of blockage of meshes of the filter cloth by solid particles arises in the same manner as in the conventional rotary drum screen using filter cloth on the screen surface.
Thus, in the prior art rotary drum type device for separating solid particles from a liquid which uses a wedge wire on the screen surface, it becomes necessary to attach a complex and large device for removing blocking of meshes of filter cloth or, alternatively, to separate solid particles by adding an expensive coagulating agent to a liquid to be treated and thereby coagulate solid particles into particles of a larger diameter which can be caught at the slits of the wedge wire.
It is, therefore, a first object of the invention to eliminate the disadvantage of the prior art rotary drum type device for separating solid particles from a liquid using a filter cloth and the prior art rotary drum type device using a wedge wire and provide a novel rotary drum type device for separating solid particles from a liquid which has less tendency to causing blockage than a filter cloth and is capable of removing blockage easily when it occurs and moreover capable of separating super fine solid particles without need for addition of an expensive coagulating agent.
The conventional rotary screen drum using a wedge wire as its screen surface is generally manufactured in the following manner.
An apparatus for manufacturing the conventional rotary screen drum using a wedge wire as its screen surface includes, as shown in FIG. 18, a screw b disposed to be rotated in a bed a. A slide c is in threaded engagement with the screw b and a carriage d is fixed to the slide c in such a manner that the carriage d can move along rails e laid on the bed a. A disk-like front support rod holder g is rotatably supported on a bearing f provided in the carriage d. An electric resistance type welder k is fixedly mounted on the bed a behind the front support rod holder g. In the rear of the welder k is rotatably disposed a disk-like rear support rod holder i. A plurality of support rods j are generally cylindrically arranged in the circumferential direction at a predetermined interval and these support rods j are held at either end thereof by the support rod holders g and i. By driving and rotating the screw b in one direction by an unillustrated drive mechanism, the carriage d is moved in direction A on the rails e. Simultaneously, a wedge wire 1 is fed from a wedge wire supplier h (shown in FIG. 18B) and wound spirally on the cylindrically arranged support rods j. A part of wedge wire wound on the support rods j is shown in a section. The wound wedge wire 1 and the support rods j are welded to each other at their crossing points by the electric resistance welder k.
This electric resistance type welder k has, as shown in FIG. 19, an upper electrode k-1 disposed outside of the wedge wire 1 and a lower electrode k-2 formed with openings through which the support rods j are supported. The upper electrode k-1 is in contact with the wedge wire 1 and the lower electrode k-2 is in contact with the support rods j. The wedge wire 1 has its smooth side directed radially outwardly and its apex opposite to the smooth surface in abutting contact with the radially outwardly projecting portion of each support rod j. FIG. 19 shows a part of the spirally wound wedge wire 1 in section.
In this prior art apparatus for manufacturing a rotary screen drum, there is usually distance of 500 mm to 600 mm between the axis X of the screw b and the axis Y of the screen drum. This distance causes an error in feeding of the screen drum in the direction of A due to deflection of the carriage d and a gap in feeding. More specifically, according to this device, an accurate slit of the wedge wire 1 can be formed on condition that feeding of the screw b in the direction A is accurately transmitted to feeding of the screen in the direction A. In actuality, however, due to weight of the screen and also to friction caused by sliding of the support rods j through an opening k-3 of the welder k when the screen is fed in the direction A, the vertical axis of Z of the carriage d is deflected to the axis Z'. Therefore, the carriage d is not fed in the direction A with its vertical axis Z exactly crossing normally to the axis X of the screw b. Therefore, the inclination Z-Z' of the vertical axis Z of the carriage b causes an error in transmission of feeding of the screw b to feeding of the screen. Consequently, it is extremely difficult to achieve an accurate feeding which is required to realize a super fine slit width of 150 microns or less.
Further, according to welding by the conventional electric resistance type welder, electric currents of a large value flow in each of the wedge wire 1 and the support rods j and welding is achieved by utilizing a high heat which is produced when the apex of the wedge wire 1 is brought into contact with the projecting portion of the support rod j. Thus, the wedge wire 1 and the support rods j are repeatedly subjected to a highly heated state and a cooled state during the welding process which causes distortion due to heat. This heat distortion causes a subtle error in the width of slit m of the wedge wire 1 wound on the support rods j and, for this reason also, it is impossible to realize a super fine slit of 150 microns or less by the prior art apparatus.
It is, therefore, a second object of the present invention to provide a method for manufacturing a rotary screen drum suitable for use in the rotary drum type device for separating solid particles from a liquid achieving the first object of the invention.